GPS uses satellites in space to locate objects on earth. With GPS, signals from the satellites arrive at a GPS receiver and are used to determine the position of the GPS receiver. Currently, two types of GPS measurements corresponding to each correlator channel with a locked GPS satellite signal are available for civilian GPS receivers. The two types of GPS measurements are pseudorange, and integrated carrier phase for two carrier signals, L1 and L2, with frequencies of 1.5754 GHz and 1.2276 GHz, or wavelengths of 0.1903 m and 0.2442 m, respectively. Some receivers output a Doppler measurement which is simply the difference in two consecutive integrated carrier phase measurements. The pseudorange measurement (or code measurement) is a basic GPS observable that all types of GPS receivers can make. It utilizes the C/A or P codes modulated onto the carrier signals. The measurement records the apparent time taken for the relevant code to travel from the satellite to the receiver, i.e. the time the coded signal left the satellite according to the satellite clock minus the time it arrives at the receiver according to the receiver clock.
The carrier phase measurement is obtained by integrating a reconstructed carrier of the coded signal as it arrives at the receiver. Thus, the carrier phase measurement is also a measure of a transit time difference as determined by the time the signal left the satellite according to the satellite clock and the time it arrives at the receiver according to the receiver clock. However, because an initial number of whole cycles in transit between the satellite and the receiver when the receiver starts tracking the carrier phase of the signal is usually not exactly known, the transit time difference may be in error by the periods of a few carrier cycles, i.e. there is a whole-cycle ambiguity in the carrier phase measurement. Since the carrier frequencies are much higher and their pulses are much closer together than those of the C/A or P codes, the carrier phase measurement can be much more accurate than the code measurement. In addition, the code measurements are affected much more than the carrier phase measurements by the interference of reflected signals with the direct signal. This interference also causes the code measurements to be less accurate than the carrier phase measurements.
With the GPS measurements available, the range or distance between a GPS receiver and a satellite is calculated by multiplying a signal's travel time by the speed of light. These ranges are usually referred to as pseudoranges (false ranges) because the receiver clock generally has a significant time error which causes a common bias in the measured range. This common bias from receiver clock error is solved for along with the position coordinates of the receiver as part of the normal navigation computation. Various other factors can lead to errors or noise in the calculated range, including ephemeris error, satellite clock timing error, atmospheric effects, receiver noise and multipath error. To eliminate or reduce these errors, differential operations are typically used in GPS applications to cancel the noise factors in the pseudorange and/or carrier phase measurements resulting from these error sources. Differential GPS (DGPS) operations typically involve a base reference GPS receiver, a user GPS receiver, and a communication mechanism between the user and reference receivers. The reference receiver is placed at a known location and the known position is used to generate corrections associated with some or all of the above error factors. The corrections are supplied to the user receiver and the user receiver then uses the corrections to appropriately correct its computed position. The corrections can be in the form of corrections to the reference receiver position determined at the reference site or in the form of corrections to the specific GPS satellite clock and/or orbit. Corrections to the reference receiver position are not as flexible because, for optimum accuracy, they require that the same satellites be observed by the user receiver and the reference receiver.
The fundamental concept of Differential GPS (DGPS) is to take advantage of the spatial and temporal correlations of the errors inherent in the GPS measurements. The GPS satellite clock timing error (clock error), which appears as a bias on the pseudorange or carrier phase measurement, is perfectly correlated between the reference receiver and the user receiver. So, a DGPS system is capable of completely removing the clock error, which typically contributes about several meters of error to the user position.
The atmospheric effects are due to the GPS signal passing through the charged particles of the ionosphere and then through the water vapor in the troposphere. The effect of the ionosphere on the GPS signal is usually modeled by an ionospheric refraction model and errors in the model contribute to errors in the computed range. These errors are strongly correlated between the reference and user receivers over short distances between the two receivers, but the correlation diminishes over large distances.
Refraction of the GPS signal in the troposphere can generally be modeled to remove 90 to 95 percent of the tropospheric effects. The residual tropospheric errors can be reduced by the use of a DGPS when the user does not roam across large distances, because the correlation of tropospheric refraction error typically disappears as soon as the user is a few tens of kilometers away from the reference receiver. So the use of the DGPS by itself does not produce meaningful reductions of the residual tropospheric refraction error. Sometimes, the uncorrelated error at the reference receiver can even worsen the situation by introducing additional error into the computed user position.
The ephemeris or satellite orbital error can be modeled as having along-track, cross track, and radial error vectors. The satellite orbital error can be reduced by using the DGPS system. However, the reduction is somewhat limited because the correlation in the satellite orbital error gradually reduces as the separation between the reference and user receivers increases. The correlation is largely diminished over continental distances.
Receiver noise and multipath (reflected signal) effects are generally uncorrelated between the reference and user receivers. These error effects are sometimes amplified by the use of DGPS systems.
To overcome the inaccuracy of the DGPS system in wide-area applications, various wide area DGPS (WADGPS) techniques have been developed. The WADGPS includes a network of multiple reference stations in communication with a computational center or hub. Error corrections are computed at the hub based upon the known locations of the reference stations and the measurements taken by them. The computed error corrections are then transmitted to users via a communication link such as satellite, phone, or radio.
In some cases, raw data such as the measurements and positions of the reference receivers are supplied to the user receiver(s) rather than the corrections. The user receiver can select the data from a particular reference station or form corrections from a weighted combination of the data from the multiple reference stations.
By using multiple reference stations, WADGPS provides more accurate estimates of the error corrections. However, the use of multiple reference stations also makes computation of the error corrections more complicated and different error factors can alias into each other, destroying the correlations inherent in the GPS measurements.